27 research outputs found
Aharonov-Bohm interferences from local deformations in graphene
One of the most interesting aspects of graphene is the tied relation between
structural and electronic properties. The observation of ripples in the
graphene samples both free standing and on a substrate has given rise to a very
active investigation around the membrane-like properties of graphene and the
origin of the ripples remains as one of the most interesting open problems in
the system. The interplay of structural and electronic properties is
successfully described by the modelling of curvature and elastic deformations
by fictitious gauge fields that have become an ex- perimental reality after the
suggestion that Landau levels can form associated to strain in graphene and the
subsequent experimental confirmation. Here we propose a device to detect
microstresses in graphene based on a scanning-tunneling-microscopy setup able
to measure Aharonov-Bohm inter- ferences at the nanometer scale. The
interferences to be observed in the local density of states are created by the
fictitious magnetic field associated to elastic deformations of the sample.Comment: Some bugs fixe
Room-temperature ferromagnetism in graphite driven by 2D networks of point defects
Ferromagnetism in carbon-based materials is appealing for both applications
and fundamental science purposes because carbon is a light and bio-compatible
material that contains only s and p electrons in contrast to traditional
ferromagnets based on 3d or 4f electrons. Here we demonstrate direct evidence
for ferromagnetic order locally at defect structures in highly oriented
pyrolytic graphite (HOPG) with magnetic force microscopy and in bulk
magnetization measurements at room temperature. Magnetic impurities have been
excluded as the origin of the magnetic signal after careful analysis supporting
an intrinsic magnetic behavior of carbon. The observed ferromagnetism has been
attributed to originate from unpaired electron spins localized at grain
boundaries of HOPG. Grain boundaries form two-dimensional arrays of point
defects, where their spacing depends on the mutual orientation of two grains.
Depending on the distance between these point defects, scanning tunneling
spectroscopy of grain boundaries showed two intense split localized states for
small distances between defects (< 4 nm) and one localized state at the Fermi
level for large distances between defects (> 4 nm).Comment: 19 pages, 5 figure
Topological Quantum Phase Transition in Synthetic Non-Abelian Gauge Potential
The method of synthetic gauge potentials opens up a new avenue for our
understanding and discovering novel quantum states of matter. We investigate
the topological quantum phase transition of Fermi gases trapped in a honeycomb
lattice in the presence of a synthetic non- Abelian gauge potential. We develop
a systematic fermionic effective field theory to describe a topological quantum
phase transition tuned by the non-Abelian gauge potential and ex- plore its
various important experimental consequences. Numerical calculations on lattice
scales are performed to compare with the results achieved by the fermionic
effective field theory. Several possible experimental detection methods of
topological quantum phase tran- sition are proposed. In contrast to condensed
matter experiments where only gauge invariant quantities can be measured, both
gauge invariant and non-gauge invariant quantities can be measured by
experimentally generating various non-Abelian gauges corresponding to the same
set of Wilson loops
Strain-induced Evolution of Electronic Band Structures in a Twisted Graphene Bilayer
Here we study the evolution of local electronic properties of a twisted
graphene bilayer induced by a strain and a high curvature. The strain and
curvature strongly affect the local band structures of the twisted graphene
bilayer; the energy difference of the two low-energy van Hove singularities
decreases with increasing the lattice deformations and the states condensed
into well-defined pseudo-Landau levels, which mimic the quantization of massive
Dirac fermions in a magnetic field of about 100 T, along a graphene wrinkle.
The joint effect of strain and out-of-plane distortion in the graphene wrinkle
also results in a valley polarization with a significant gap, i.e., the
eight-fold degenerate Landau level at the charge neutrality point is splitted
into two four-fold degenerate quartets polarized on each layer. These results
suggest that strained graphene bilayer could be an ideal platform to realize
the high-temperature zero-field quantum valley Hall effect.Comment: 4 figure
Properties of Graphene: A Theoretical Perspective
In this review, we provide an in-depth description of the physics of
monolayer and bilayer graphene from a theorist's perspective. We discuss the
physical properties of graphene in an external magnetic field, reflecting the
chiral nature of the quasiparticles near the Dirac point with a Landau level at
zero energy. We address the unique integer quantum Hall effects, the role of
electron correlations, and the recent observation of the fractional quantum
Hall effect in the monolayer graphene. The quantum Hall effect in bilayer
graphene is fundamentally different from that of a monolayer, reflecting the
unique band structure of this system. The theory of transport in the absence of
an external magnetic field is discussed in detail, along with the role of
disorder studied in various theoretical models. We highlight the differences
and similarities between monolayer and bilayer graphene, and focus on
thermodynamic properties such as the compressibility, the plasmon spectra, the
weak localization correction, quantum Hall effect, and optical properties.
Confinement of electrons in graphene is nontrivial due to Klein tunneling. We
review various theoretical and experimental studies of quantum confined
structures made from graphene. The band structure of graphene nanoribbons and
the role of the sublattice symmetry, edge geometry and the size of the
nanoribbon on the electronic and magnetic properties are very active areas of
research, and a detailed review of these topics is presented. Also, the effects
of substrate interactions, adsorbed atoms, lattice defects and doping on the
band structure of finite-sized graphene systems are discussed. We also include
a brief description of graphane -- gapped material obtained from graphene by
attaching hydrogen atoms to each carbon atom in the lattice.Comment: 189 pages. submitted in Advances in Physic
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Landau Levels and Edge States in Graphene with Strong Spin-Orbit Coupling
We investigate the electronic properties of graphene in a magnetic and a strain-induced pseudo-magnetic field in the presence of strong spin-orbit interactions (SOI). For a homogeneous field we provide analytical results for the Landau level eigenstates for arbitrary intrinsic and Rashba SOI, including also the effect of a Zeeman field. We then study the edge states in a semi-infinite geometry in the absence of the Rashba term. We find that, for a critical value of the magnetic field, a quantum phase transition occurs, which separates two phases both with spin-filtered helical edge states but with opposite direction of the spin current. Finally,we discuss magnetic waveguides with inhomogeneous field profiles that allow for chiral snake orbits. Such waveguides are practically immune to disorder-induced backscattering, and the SOI provides non-trivial spin texture to these modes
First-Principles Study of the Electronic and Magnetic Properties of Defects in Carbon Nanostructures
Understanding the magnetic properties of graphenic nanostructures is
instrumental in future spintronics applications. These magnetic properties are
known to depend crucially on the presence of defects. Here we review our recent
theoretical studies using density functional calculations on two types of
defects in carbon nanostructures: Substitutional doping with transition metals,
and sp-type defects created by covalent functionalization with organic and
inorganic molecules. We focus on such defects because they can be used to
create and control magnetism in graphene-based materials. Our main results are
summarized as follows: i)Substitutional metal impurities are fully understood
using a model based on the hybridization between the states of the metal
atom and the defect levels associated with an unreconstructed D carbon
vacancy. We identify three different regimes, associated with the occupation of
distinct hybridization levels, which determine the magnetic properties obtained
with this type of doping; ii) A spin moment of 1.0 is always induced by
chemical functionalization when a molecule chemisorbs on a graphene layer via a
single C-C (or other weakly polar) covalent bond. The magnetic coupling between
adsorbates shows a key dependence on the sublattice adsorption site. This
effect is similar to that of H adsorption, however, with universal character;
iii) The spin moment of substitutional metal impurities can be controlled using
strain. In particular, we show that although Ni substitutionals are
non-magnetic in flat and unstrained graphene, the magnetism of these defects
can be activated by applying either uniaxial strain or curvature to the
graphene layer. All these results provide key information about formation and
control of defect-induced magnetism in graphene and related materials.Comment: 40 pages, 17 Figures, 62 References; Chapter 2 in Topological
Modelling of Nanostructures and Extended Systems (2013) - Springer, edited by
A. R. Ashrafi, F. Cataldo, A. Iranmanesh, and O. Or